Which is Greener: Wool or Cotton?

When it comes to garments and fabric, which is greener: wool or cotton? Slate's Green Lantern tackles the question, doing some analysis and ultimately coming up with...well, it depends.

It's an apples to oranges comparison, notes the Lantern -- one comes from sheep, the other grows in the ground -- and, on top of that, there are a lot of environmental impacts to consider with both. Wool is a renewable resource, but sheep belch 20 to 30 liters of climate-changing methane per day; cotton's organic variety is grown without petrochemical fertilizers and pesticides, but, grown conventionally, the plant is a toxic mess. Hmm.

Okay, the wool first. In New Zealand, home to 45 million sheep (to under 5 million people), more than half of the nation's greenhouse gas emissions come from their livestock; the methane that the sheep so conspicuously add to the atmosphere has a global warming potential of 21, compared to (a much smaller) 1 for carbon dioxide.

Water, the world's most precious resource, plays a big role, too, from raising the sheep to cleaning the fiber; it takes approximately 500,000 liters of water to manufacture a metric ton of wool, though cotton requires 2,500 liters of water for just one t-shirt, and that's just for its growth.

And when it comes to fertilizers, the beat goes on with cotton. "In Australia, where nitrous oxide emissions have increased 130 percent since 1990 due to fertilizer usage, it's estimated that a third of the nitrogen applied to cultivated fields is lost before serving any purpose. The furrows of cotton fields are particularly egregious emitters; in 2006, researchers from the Queensland University of Technology found that each acre of furrowed land accounted for 13.8 ounces of released nitrous oxide."

Ultimately, the Lantern sides with cotton, but it wasn't an easy choice. We could probably argue semantics all day long -- carbon footprint vs. organic agriculture vs. lifetime energy and cleaning use -- but it underscores the decisions required to make such a decision. Which is more important: keeping pesticides out of the ground or greenhouse gases out of the air? Paying for organic or exposing workers to pounds of fertilizer? The choice is yours. ::Slate